The field of art to which this invention pertains is the dehydrogenation of hydrocarbons in multiple reaction zones that use on stream catalyst replacement.
The dehydrogenation of hydrocarbons is an important commercial hydrocarbon conversion process because of the existing and growing demand for dehydrogenated hydrocarbons for the manufacture of various chemical products such as detergents, high octane gasolines, oxygenated gasoline blending components, pharmaceutical products, plastics, synthetic rubbers, and other products which are well known to those skilled in the art. One example of this process is the dehydrogenation of isobutane to produce isobutylene which can be polymerized to provide tackifying agents for adhesives, viscosity-index additives for motor oils, and impact-resistant and antioxidant additives for plastics. Another example of the growing demand for isobutylene is the production of oxygen-containing gasoline blending components which are being mandated by the government in order to reduce air pollution from automotive emissions.
Those skilled in the art of hydrocarbon conversion processing are well versed in the production of olefins by means of catalytic dehydrogenation of paraffinic hydrocarbons. In addition, many patents have issued which teach and discuss the dehydrogenation of hydrocarbons in general. For example, U.S. Pat. No. 4,430,517 (Imai et al) discusses a dehydrogenation process and catalyst for use therein.
Most catalysts for the dehydrogenation of hydrocarbons are susceptible to deactivation over time. Deactivation will typically occur because of an accumulation of deposits that block active pore sites or catalytic sites on the catalyst surface. Where the accumulation of coke deposits causes the deactivation, reconditioning the catalyst to remove coke deposits restores the activity of the catalyst. Coke is normally removed from the catalyst by contact of the coke-containing catalyst with an oxygen-containing gas at a high enough temperature to combust or remove the coke in a regeneration process. In a moving bed process, the regeneration process is carried out by removing catalyst from the vessel in which the hydrocarbon conversion is taking place and transporting the catalyst to a separate regeneration zone for coke removal. Arrangements for continuously or semi-continuously removing catalyst particles from a bed in a reaction zone for coke removal in a regeneration zone are well known. U.S. Pat. No. 3,652,231 describes a continuous catalyst regeneration process which is used in conjunction with the catalytic reforming of hydrocarbons, the teachings of which are hereby incorporated by reference. In the reaction zone of U.S. Pat. No. 3,652,231, the catalyst is transferred under gravity flow by removing catalyst from the bottom of the reaction zone and adding catalyst to the top while reactants flow cross currently through a radial flow bed.
In the past, flow-related phenomena have limited mass flow and fluid velocity through the radial flow beds. One phenomenon, known as xe2x80x9cpinning,xe2x80x9d inhibits catalyst transfer in many reactor arrangements. Pinning occurs when the flow of fluid at sufficient velocity blocks the downward movement of catalyst. Pinning is a function of the gas composition, the gas velocity, the physical characteristics of the catalyst, and the physical characteristics of the flow channel through which the catalyst must move. As the gas flows through the channels that retain the catalyst, the gas impacts the catalyst particles and raises intergranular friction between the particles. When the vertical component of the frictional forces between the particles overcomes the force of gravity on the particles, the particles become pinned. As the flow path length of gas through the catalyst particles becomes longer, the forces on the particles progressively increase from the outlet to the inlet of the flow channel.
Another flow-limiting phenomena is called xe2x80x9cvoid blowingxe2x80x9d. Void blowing occurs when the gas velocity displaces from a surface of the catalyst bed across which it flows from the screen or other retaining elements, thereby creating a void space. The rapid circulation or churning of catalyst over the free surface of the void space can abrade or break catalyst particles, resulting in the production of reduced size particles or fines. This fine material can plug the catalyst bed, exacerbate the void blowing problem, or accumulate in other process piping or equipment in a manner that interferes with the continued effective operation of the process.
As technology has improved and problems such as pinning and void blowing have been better understood, it has been possible to increase the fluid velocity through the radial flow beds of existing reaction zones that were previously limited by these fluid flow phenomena. Increasing the velocity or throughput is of course desirable because it permits an increase in capacity with only minor operating changes to the existing equipment. However, it has been found that further increases in the capacity of many existing dehydrogenation units cannot only be obtained with decreases in conversion or selectivity of the products produced. The limitations in conversion and/or selectivity are related to the reduced time that results from the higher fluid velocity through the catalyst beds. The catalyst contact time is typically expressed in terms of the liquid hourly space velocity (LHSV) of the feed through the catalyst bed. Higher LHSV""s may be compensated for, to some extent, by an increased reaction temperature which raises the catalyst activity. Although compensating for higher throughput by increasing the reactor temperatures may maintain conversion levels, it is typically at the expense of lower selectivity and increased coke production on the catalyst.
The most direct way to overcome the problems of space velocity limitations is to add more catalyst to the process. Increasing the catalyst volume is readily accomplished in the design stage for a new unit. Unfortunately for existing units, adding additional catalyst could require expensive modification or replacement of all of the reactors and the associated piping for the delivery of reactants and the transfer of catalyst between the reactors.
Accordingly, it is an objective of this invention to increase the throughput of existing dehydrogenation reactors with only limited modifications to the reaction zone and the associated piping.
It has now been surprisingly discovered that it is possible to increase throughput in a dehydrogenation process without sacrificing the conversion or selectivity by only increasing the catalyst volume in the last reaction zone and that it is possible to increase the catalyst volume of the last reaction zone in a simple and relatively inexpensive manner. Adding catalyst to the last reaction zone effectively decreases the overall LHSV for the feed stream at higher throughput rates. As a result, conversion is maintained without increasing the temperature in any of the individual reactors and thereby avoiding any fall off in selectivity or increase in coke production. Achieving these results by only requiring a simple modification of the last reaction zone makes the operation of such dehydrogenation processes at higher throughput highly cost effective. The method applies to multiple reactors wherein the last reactor is in a side-by-side arrangement to the upstream reactors. Furthermore, the modification of the last reaction zone may be simplified by the arrangement of many reactors which often include a flange connection at a location that may be beneficially used to extend the catalyst volume of the radial flow bed.
One embodiment of the present invention may be characterized as a method of increasing the throughput of a process for the dehydrogenation of hydrocarbons. The process passes a combined dehydrogenation feed into contact with a catalyst bed in a first reactor, passes the effluent from the first reactor into contact with a catalyst bed in at least one intermediate reactor, and passes an intermediate effluent from the last of the at least one intermediate reactor into contact with a final catalyst bed in a final reactor. The process effects catalyst circulation in at least the last reactor by adding catalyst to the top of the final catalyst bed and withdraws catalyst from the bottom of the final catalyst bed in a final reactor while contacting the intermediate effluent in the final catalyst bed of the final reactor. The final reactor is located to the side of the intermediate reactor from which it receives the intermediate effluent and the catalyst bed of the final reactor retains catalyst between an inner and outer screen in an arrangement for the radial flow of reactants. The method includes splitting the vessel wall of the last reactor at a disconnection point above the middle of the inner and outer screens to define a lower reactor section that retains the inner and outer screens and a disconnected upper reactor section that retains equipment for supplying catalyst to the final catalyst bed. The disconnected upper section is at least temporarily removed. The inner and outer screens are extended by a distance that will increase the catalyst volume in the process by at least 5%. A new section of vessel wall installed above the disconnection point encloses the extended inner and outer screens with a closing upper section of the final reactor that retains equipment for supplying catalyst to the final catalyst bed.
Extending the inner and outer screens increases the volume of the catalyst with a corresponding decrease in the LHSV of the process. The catalyst volume will usually be increased by an amount that will keep the LHSV of the process below about 4 hrsxe2x88x921. Increasing the catalyst volume in the last reaction will also result in the local LHSV of the first reactor and any intermediate reactor being greater than the LHSV of the final reactor. The LHSV of the final reactor, once the screens have been extended, will usually be at least 5 hrsxe2x88x921 less than the LHSV of the other reactors. Of course, any decrease in the LHSV will be limited by the amount of catalyst that can be added to the modified reactor. Preferably, the final reactor will increase the catalyst volume by 10% and, more preferably, the catalyst volume will increase by 15% or more. Structural limitations such as foundation support may control the amount of catalyst that may be added to the final reaction zone by extension of the inner and outer screens.
The splitting of the last reactor may take place at any convenient disconnection point. A disconnection point at or above the inner and outer screens is preferred in order to minimize the disturbance of existing screen supports and related instrumentation or other miscellaneous piping that may be connected to the reactor vessel. The upper section of the reactor that is removed for access to the screens may be reused or replaced with a new reactor section that contains an additional section of external vessel wall. Preferably, the upper reaction section will be reused along with the equipment for the delivery of catalyst that is typically included therewith.
It is particularly convenient to reuse the upper section when the existing reactor has an upper section that is flanged to the lower section. In accordance with such an arrangement, this invention comprises a method of increasing the capacity of a process for the dehydrogenation of hydrocarbons. The process passes a combined dehydrogenation feed into contact with a first catalyst bed in a first reactor at a first LHSV through the first catalyst bed, passes the effluent from the first reactor into contact with an intermediate catalyst bed in at least one intermediate reactor at an intermediate LHSV through the intermediate catalyst bed, and passes an intermediate effluent from the last of the at least one intermediate reactor into contact with a final catalyst bed in a final reactor at a final LHSV. The process adds catalyst to the top of the final catalyst bed and withdraws catalyst from the bottom of the final catalyst bed in the final reactor while contacting the intermediate effluent in the final catalyst bed of the final reactor. The final reactor is located to the side of the intermediate reactor from which it receives the intermediate effluent and the catalyst bed of the final reactor retains catalyst between an inner and outer screen in an arrangement for the radial flow of reactants. The method splits the vessel wall of the last reactor above the top of the inner and outer screens to create an upper reactor section that retains catalyst supply equipment for supplying catalyst to the final reactor bed and a lower reactor section that retains the inner and outer screens. The upper section is temporarily removed and the inner and outer screens are extended by a distance that will retain sufficient catalyst to reduce the LHSV of the last reactor below the lowest LHSV of the first reactor and all of the intermediate reactors. The method also inserts an intermediate section of vessel wall between the upper and lower reactor sections in a sealed arrangement to enclose the extended inner and outer screens and to position the catalyst supply equipment for delivery of catalyst to the final reactor bed. The split is preferably made across a flanged connection and the new section of vessel wall has flanged ends for attachment to the existing flanges of the upper and lower reactor sections.
Other embodiments of the present invention encompass further details of preferred process conditions and reactor modification methods.